Silva made the surprise nine-word comment after Spain's 1-0 win over Belarus on Sunday night, telling reporters: "Iker's a great goalkeeper and for us the best."
"There's nothing more to say," he added, accorded to ESPN.
Casillas was given the nod over De Gea for Spain's Euro 2016 qualifier by Spain head coach Vicente del Bosque, who has always remained fiercely loyal to the players who have helped him win silverware with La Roja.
De Gea is expected to replace Casillas at Real Madrid next season, but Del Bosque - with the backing of the Spain players, it seems - has kept faith with the under-fire 34-year-old who has been in steep decline since 2013.
Casillas and De Gea's relationship
Footage obtained by Spanish media last week suggests the relationship between Casillas and De Gea is, at best, frosty. One video showed the two goalkeepers pulling up for training at the same time, before Casillas appeared to blank his younger compatriot.
But Casillas told reporters last month that De Gea would be welcomed at the Bernabeu, adding that the 24-year-old is a "great guy".
"If De Gea does come here, he’ll be welcomed," Casillas was quoted as saying by the Daily Mail. "Competition is healthy and there is not a clause in my contract which states that I have to be No.1. We’ll have to compete for a place like we do with Spain.
"De Gea is a great guy and a goalkeeper for the present and the future. He can handle pressure and the experience he’s had at Manchester United will have served him well.
"Our president Florentino Perez once asked me if it were up to me, which keeper would I sign. I prefer Spanish keepers – Sergio Rico, (Sergio) Asenjo and De Gea. Right now, De Gea is the expert out of the three."
De Gea is a better keeper than Casillas
De Gea appreciates Casillas' standing in the game - he is a legend of both Spain and Real Madrid - and is content to play the role of under-study for the time being.
However, it seems almost inexplicable that Del Bosque isn't using De Gea for Spain's important matches.
Few would argue that Casillas is, at this moment in time, better than De Gea who is now widely considered as one of the best three goalkeepers in world football alongside Bayern Munich's Manuel Neuer and Chelsea's Thibaut Courtois.
There's a reason why Real Madrid are determined to sign De Gea this summer: Casillas is no longer the world-class keeper he once was.
Silva may have been attempting to stick up for a player who has given so much to Spanish football over the years - but he may have inadvertently upset De Gea's ego in the process.
Is David Silva deliberately attempting to anger David de Gea with this comment? Have your say in the comments below!